Vesicular systems, including liposomes, have been incorporated into many cosmetic formulations. Liposomes are colloidal particles, typically comprising mainly phospholipids and cholesterol. The lipid molecules form bilayers surrounding an aqueous core. Both the bilayer and the core can be used to entrap and present agents to the tissue with which they contact (for instance, the skin). Most studies describe a non-specific targeting effect whereby vesicles allow accumulation of ingredients in the stratum corneum or other upper skin layers. A number of studies have reported enhanced delivery of a variety of pharmaceutical substances, including triamcinolone, methotrexate, hydrocortisone, tretinoin, tacrolimus, rhodamine, cyclosporin and antiandrogens into and through the skin using liposomal formulations.
Transdermal delivery using liposomes as carriers has been illustrated in a few cases where the entrapped drug was able to cross all skin layers (Kato et al., J Pharm. Pharmacol, 1987 39(5): p. 399-400). A number of studies have demonstrated that the vesicle composition, for instance, the inclusion of skin lipids (Fresta et al., J. Drug Target, 1996 4(2): p. 95-101), positively charged lipids (Kitagawa et al., Chem Pharm Bull (Tokyo), 2006 54(2): p. 242-4), and presence of surfactants in the bilayer (Hofland et al., Pharmaceutical Research, 1994 11(5): p. 659-664) may have an effect on substance permeation.
The state of the lipid bilayers of the vesicles, namely the liquid crystal phase or gel phase, also affects dermal and transdermal delivery: liquid crystal state vesicles are thought to be more effective. Such results have been confirmed in vivo (Ogiso et al., Journal of Pharmaceutical Sciences, 1996 85(1): p. 57-64). Other physico-chemical properties, such as particle charge, particle size and lamellarity may also influence the degree of substance transport.
Vesicles other than liposomes have been devised with the aim of improving transdermal and topical delivery of substances. Examples of these include vesicles made of non-ionic surfactants (Niosomes™) (Schreier et al., Journal of Controlled Release, 1994 30(1): p. 1-15), vesicles containing a high percentage of ethanol (ethosomes) (Touitou et al., Drug Development Research, 2000b 50(3-4): p. 406-415) and ultraflexible vesicles otherwise known as transfersomes. Polymersomes (vesicles formed from amphiphilic block copolymers) are the polymeric equivalent of liposomes and because of their macromolecular nature, are known to be much more robust and stable than their lipid counterparts (Discher et al., Science 284 (1991), 1143-6). In addition, their macromolecular nature also allows a very effective tuning of the membrane thickness. Recently, pH sensitive polymersomes have been developed which are able to encapsulate and deliver plasmid DNA to the cell cytosol (Lomas et al., 2007 Adv. Mater 19, 4238-4234).
Liaw et al in Journal of Controlled Release 68 (2000) 273-282, describe percutaneous fentanyl delivery using poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) gels. The polymer forms micelles which are able to penetrate nude mouse skin within 24 hours.